Light emitting apparatus, method for driving the light emitting apparatus, and display apparatus including the light emitting apparatus

ABSTRACT

A light emitting apparatus comprises a light emitting section for emitting light, a color of the light being changed with a value of a driving current, and a driving section for driving the light emitting section so that the light emitting section emits light having a desired color and a desired intensity, by generating the driving current based on a signal designating the desired color and a signal designating the desired intensity and by applying the driving current to the light emitting section.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.11/644,668, filed Dec. 22, 2006, which is a continuation of U.S.application Ser. No. 10/631,932, filed Jul. 30, 2003, now U.S. Pat. No.7,180,487, which is a continuation of U.S. application Ser. No.09/711,353, filed Nov. 9, 2000, now U.S. Pat. No. 6,628,249, whichclaims the priority of Japanese Patent Application No. 11-322044, filedNov. 12, 1999, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light emitting apparatus and a methodfor driving the light emitting LED apparatus. The present invention alsorelates to a display apparatus including the light emitting apparatus.

2. Description of the Related Art

FIG. 20 illustrates a conventional LED driving apparatus. In theconventional LED driving apparatus, a driving current i corresponding toan output establishment value p is applied from a power source 191 to anLED device 192 to drive the LED device 192. The emission intensity ofthe LED device 192 is controlled by changing the value of the drivingcurrent i. In general, the color of light emitted from conventional LEDdevices does not depend on the value of a driving current. Thus, theemission intensity of conventional LED devices is changed by control ofthe driving current value while maintaining the color of light emittedfrom the LED devices.

The emission intensity of conventional LED devices can be solely changedby control of the driving current value, since the light color of theconventional LED devices is not affected by a change in the drivingcurrent value. However, there is another type of LED device having alight color which is altered by a change in the driving current value.When such an LED device is driven by a driving apparatus as shown inFIG. 20, the light color of the LED device is changed along with theemission intensity control thereof. Thus, a desired emission intensitycontrol cannot be conducted.

Conventional AlGaAs semiconductor light emitting devices have aphenomenon in which the wavelength of light emitted therefrom becomeslonger as a driving current is increased (red shift phenomenon). The redshift phenomenon occurs as follows. The light emitting device is heatedby supplied power, and the temperature of the device is proportional tothe average supplied power. The increased temperature causes the bandgapof the active layer to be reduced, so that the wavelength of lightemitted from the device becomes longer (red shift). In other words, theemission intensity and emission wavelength of the light emitting devicehaving the red shift phenomenon are determined by the average powerapplied to the device. It is not possible to separately control theemission wavelength and emission power of the AlGaAs semiconductor lightemitting device using two parameters (peak value and average power) of adriving current pulse.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a light emittingapparatus comprises: a light emitting section for emitting light, acolor of the light being blue shifted with a value of a driving current;and a driving section for driving the light emitting section so that thelight emitting section emits light having a desired color and a desiredintensity, by generating the driving current based on a signaldesignating the desired color and a signal designating the desiredintensity and by applying the driving current to the light emittingsection.

In one embodiment of this invention, the driving section comprises: aconverting section for converting the color-designating signal to asignal designating a current value in accordance with current versusemission wavelength characteristics of the light emitting section; acalculating section for calculating a signal designating a duty based onthe intensity-designating signal and the current value-designatingsignal, in such a manner that the product of a current value designatedby the current value-designating signal and a duty corresponds to theintensity designated by the intensity-designating signal; and a sectionfor generating, as the driving current, a pulse current having thecurrent value and the duty in accordance with the respective currentvalue-designating signal and duty-designating signal.

In one embodiment of this invention, the light emitting section is anLED device.

In one embodiment of this invention, the light emitting section is anLED bullet comprising: an LED device; and a fluorescent excited by lightemitted by the LED device to emit light.

In one embodiment of this invention, when the driving current value ischanged, a change in an emission wavelength of the LED bullet is largerthan a change in an emission wavelength of the LED device.

In one embodiment of this invention, the light emitting apparatuscomprises: a plurality of the light emitting sections; and a pluralityof the driving sections respectively corresponding to the plurality ofthe light emitting sections. A color and intensity of light emitted byeach of the plurality of the light emitting sections is controlled by acorresponding one of the plurality of the driving sections.

In one embodiment of this invention, the color-designating signal is acolor signal for designating a color of light to be emitted by the LEDdevice, and the intensity-designating signal is an intensity signal fordesignating an intensity of the light to be emitted by the LED device.

In one embodiment of this invention, the color-designating signal is achromaticity signal for designating a color of light to be emitted bythe LED bullet, and the intensity-designating signal is a luminancesignal for designating an intensity of the light to be emitted by theLED bullet.

In one embodiment of this invention, the LED device comprises: an InGaNactive layer; and an AlGaN layer having two layers, provided on theInGaN active layer. One of the two layers is a first AlGaN layercontacting the InGaN active layer, and the other of the two layers ofthe AlGaN layer is a second AlGaN layer provided on the first AlGaNlayer, the first AlGaN layer is produced at substantially the samegrowth temperature as the InGaN active layer, and the second AlGaN layeris produced at a growth temperature higher than the growth temperatureof the first AlGaN layer.

In one embodiment of this invention, the LED device comprises: an InGaNactive layer; and an AlGaN layer having two layers, provided on theInGaN active layer. One of the two layers is a first AlGaN layercontacting the InGaN active layer, and the other of the two layers ofthe AlGaN layer is a second AlGaN layer provided on the first AlGaNlayer, the first AlGaN layer is produced at substantially the samegrowth temperature as the InGaN active layer, and the second AlGaN layeris produced at a growth temperature higher than the growth temperatureof the first AlGaN layer.

According to another aspect of the present invention, a method fordriving a light emitting apparatus is provided. The apparatus comprises:a light emitting section for emitting light, a color of the light beingblue shifted with a value of a driving current; and a driving sectionfor driving the light emitting section. The method comprises the stepsof: receiving a signal designating a desired color and a signaldesignating a desired intensity; generating the driving current based onthe color-designating signal and the intensity-designating signal; andapplying the driving current to the light emitting section so that thelight emitting section emits light having the desired color and thedesired intensity.

In one embodiment of this invention, the method further comprises thesteps of: converting the color-designating signal to a signaldesignating a current value in accordance with current versus emissionwavelength characteristics of the light emitting section; calculating asignal designating a duty based on the intensity-designating signal andthe current value-designating signal, in such a manner that the productof a current value designated by the current value-designating signaland the duty corresponds to the intensity designated by theintensity-designating signal; and generating as the driving current apulse current having the current value and the duty in accordance withthe respective current value-designating signal and duty-designatingsignal.

In one embodiment of this invention, the light emitting section is anLED device.

In one embodiment of this invention, the light emitting section is anLED bullet comprising: an LED device; and a fluorescent excited by lightemitted by the LED device to emit light.

In one embodiment of this invention, when the driving current value ischanged, a change in an emission wavelength of the LED bullet is largerthan a change in an emission wavelength of the LED device.

In one embodiment of this invention, the light emitting apparatuscomprises: a plurality of the light emitting sections; and a pluralityof the driving sections respectively corresponding to the plurality ofthe light emitting sections. A color and intensity of light emitted byeach of the plurality of the light emitting sections is controlled by acorresponding one of the plurality of the driving sections.

In one embodiment of this invention, the color-designating signal is acolor signal for designating a color of light to be emitted by the LEDdevice, and the intensity-designating signal is an intensity signal fordesignating an intensity of the light to be emitted by the LED device.

In one embodiment of this invention, the color-designating signal is achromaticity signal for designating a color of light to be emitted bythe LED bullet, and the intensity-designating signal is a luminancesignal for designating an intensity of the light to be emitted by theLED bullet.

In one embodiment of this invention, the LED device comprises: an InGaNactive layer; and an AlGaN layer having two layers, provided on theInGaN active layer. One of the two layers is a first AlGaN layercontacting the InGaN active layer, and the other of the two layers ofthe AlGaN layer is a second AlGaN layer provided on the first AlGaNlayer, the first AlGaN layer is produced at substantially the samegrowth temperature as the InGaN active layer, and the second AlGaN layeris produced at a growth temperature higher than the growth temperatureof the first AlGaN layer.

In one embodiment of this invention, the LED device comprises: an InGaNactive layer; and an AlGaN layer having two layers, provided on theInGaN active layer. One of the two layers is a first AlGaN layercontacting the InGaN active layer, and the other of the two layers ofthe AlGaN layer is a second AlGaN layer provided on the first AlGaNlayer, the first AlGaN layer is produced at substantially the samegrowth temperature as the InGaN active layer, and the second AlGaN layeris produced at a growth temperature higher than the growth temperatureof the first AlGaN layer.

In one embodiment of this invention, the light emitting section is anLED device, the LED device is driven by a pulse current having a currentvalue and a duty, the current value is set to a value corresponding toan emission wavelength of the LED device, and the duty is set to a valuecorresponding to an emission intensity of the LED device.

In one embodiment of this invention, the current value is set to a valuein a range in which a change in the emission wavelength of the LEDdevice is about 6 nm, and the duty is changed in accordance with theemission intensity of the LED device.

In one embodiment of this invention, the current value is controlled sothat the emission wavelength of the LED device is changed, and the dutyis controlled so that the emission intensity of the LED device issubstantially maintained constant.

In one embodiment of this invention, the light emitting section is anLED bullet comprises: an LED device; and a fluorescent excited by lightemitted by the LED device to emit light. An emission wavelength of theLED device is changed by changing the driving current value so that acolor of light emitted by the LED bullet is changed.

In one embodiment of this invention, the light emitting section is anLED bullet comprises: an LED device; and a fluorescent excited by lightemitted by the LED device to emit light. The LED device is driven by apulse current having a current value and a duty wherein the currentvalue is set to a value in accordance with current versus emissionwavelength characteristics of the LED bullet, and the duty is set to avalue in accordance with emission intensity characteristics of the LEDbullet.

According to another aspect of the present invention, a displayapparatus comprises: a plurality of light emitting sections provided ina plane, light emitted by each of the plurality of light emittingsections being blue shifted with a value of a driving current; and aplurality of driving sections each for driving a corresponding one ofthe plurality of light emitting sections so that the corresponding oneof the plurality of light emitting sections emits light having a desiredcolor and a desired intensity, by each generating the driving currentbased on a signal designating the desired color and a signal designatingthe desired intensity and by each applying the driving current to thecorresponding one of the plurality of light emitting sections.

In one embodiment of this invention, each of the plurality of lightemitting sections is an LED device.

In one embodiment of this invention, each of the plurality of lightemitting sections is an LED bullet comprising: an LED device; and afluorescent excited by light emitted by the LED device to emit light.

According to another aspect of the present invention, an LED bulletcomprises: an LED device having a light color, the light color beingblue shifted with a change in a driving current; and a fluorescentexcited by light emitted by the LED device to emit light.

Thus, the invention described herein makes possible the advantages ofproviding (1) a method for driving an LED device having a light colorwhich is blue shifted with a driving current value, in such a mannerthat the emission intensity of the LED device is changed whilemaintaining the light color of the LED device from being changed; (2) amethod for driving an LED device in which variation of the emissionintensity and light color of the LED device can be suppressed; (3) amethod for driving an LED device in which light emitted from the LEDdevice can be changed by utilizing a characteristic of the LED device inwhich the light color of the LED device is changed with a currentdriving value; (4) an apparatus for achieving the above-describedmethods; (5) an LED bullet including an LED device and a fluorescent andcapable of providing a wide range of variations in light color; and (6)a method for driving the LED bullet.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an LED apparatus according toExample 1 of the present invention.

FIG. 2 a schematic diagram illustrating a configuration of an LED deviceaccording to Example 1 of the present invention.

FIG. 3 is a graph showing current versus emission wavelengthcharacteristics of the LED device of FIG. 2.

FIG. 4 is a graph showing an emission wavelength spectrum of the LEDdevice of FIG. 2.

FIG. 5 is a graph showing the product of driving current and duty versusemission power characteristics of the LED device of FIG. 2 driven by theLED on-off circuit of the present invention shown in FIG. 1.

FIG. 6 is a schematic diagram illustrating a pulse signal generated by apulse generator of the LED on-off circuit of the present invention shownin FIG. 1.

FIG. 7 is a schematic diagram illustrating an LED apparatus according toExample 3 of the present invention.

FIG. 8 is a graph showing the current versus emission powercharacteristics of an LED device driven by the LED on-off circuit of thepresent invention shown in FIG. 7.

FIG. 9 is a schematic diagram illustrating an LED bullet according toExample 5 of the present invention.

FIG. 10 is a graph showing the excitation spectrum and emission spectrumof a fluorescent used in the LED bullet of Example 5.

FIG. 11 is a CIE standard chromaticity diagram.

FIG. 12 is a graph showing the current versus emission wavelengthcharacteristics of an LED device employed in Example 6 of the presentinvention.

FIG. 13 is a UCS chromaticity diagram.

FIG. 14 is a graph showing the excitation spectrum and emission spectrumof a fluorescent used in Example 6.

FIG. 15 is a graph showing luminance characteristics of an LED bulletemployed in Example 7 of the present invention.

FIG. 16 is a graph showing the u value versus current characteristics ofthe LED bullet in Example 7.

FIG. 17 is a schematic diagram illustrating an LED on-off circuitemployed in Example 7.

FIG. 18 is a schematic diagram illustrating forms of generated pulsesignals.

FIG. 19 is a schematic diagram illustrating a display apparatusaccording to Example 10 of the present invention.

FIG. 20 is a schematic diagram of a conventional LED on-off apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described by way ofillustrative examples with reference to the accompanying drawings.

The present invention provides a method for controlling an LED devicehaving an active layer which is an InGaN quantum well layer and anemission wavelength which is blue shifted (becomes shorter). In themethod of the present invention, the emission power and emissionwavelength of the LED device can be tuned to desired values by employinga pulse driving system. The inventors have focused on the fact that theblue shift in the LED device is determined depending on theinstantaneous amount of carriers in the active layer of the LED device.The emission wavelength of the LED device is controlled using the peakvalue of a pulse driving current. In the meantime, the emissionintensity of the LED device is controlled using the average power of thepulse driving current. Therefore, the pulse duration of the pulsedriving current is preferably as long as or longer than therecombination life span of a carrier in the active layer of the LEDdevice. This is because when the pulse duration is shorter than therecombination life span of a carrier in the active layer, the effectivecarrier density of the active layer is a value obtained by calculatingthe integral of carriers over the recombination life span of a carrier,so that the carrier amount of the active layer does not depend directlyon the peak value of each pulse. In general, the recombination life spanof a carrier in an LED device having the blue shift phenomenon of whichan active layer is made of InGaN is about 0.2 ns or more. The pulseduration of each pulse in the driving current is preferably about 0.2 nsor more, more preferably about 1 ns or more.

A major problem with the above-described blue shift is that the colortone of light emitted from the LED device is visually changed. Theobject of the present invention is to drive an LED device, where thetone of the emission intensity of the LED device is modulated by simplychanging a current amount, under conditions given by:

λ₁−λ₂≧6 nm and I ₁ <I ₂

where in the range from the minimum current value or more to the maximumcurrent value or less, λ₁ is the longest emission wavelength and I₁ isthe current value when the longest emission wavelength is obtained, andλ₂ is the shortest emission wavelength and I₂ is the current value whenthe shortest emission wavelength is obtained.

For example, the present invention is applied to the case where thedifference in emission wavelength between when the LED device is drivenby a direct current of about 0.1 mA and when the LED device is driven bya direct current of about 30 mA is about 6 nm or more. Conversely, whenthe LED device is controlled so that wavelength shift is restricted to apredetermined level, the LED apparatus should be designed so that thewavelength shift falls within the range of about 6 nm or less.

Example 1

A light emitting apparatus according to Example 1 of the presentinvention includes: a light emitting section for emitting light in whicha color of the light is blue shifted with a value of a driving current;and a driving section for driving the light emitting section so that thelight emitting section emits light having a desired color and a desiredintensity, by generating the driving current based on a signaldesignating the desired color and a signal designating the desiredintensity and by applying the driving current to the light emittingsection. In description of Example 1, the light emitting section servesas an LED device, and the driving section serves as an LED on-offcircuit.

FIG. 1 illustrates an LED apparatus according to Example 1 of thepresent invention. The LED apparatus of Example 1 includes an LED on-offcircuit 1 for outputting an LED driving current R based on an intensitysignal p and a color signal c, and an LED device 104 having a lightcolor which is changed with a change in a driving current value.

The LED on-off circuit 1 includes: a color signal-current value signalconverter 102 for converting the color signal c to a current valuesignal i; a calculation processor 101 for calculating and outputting aduty signal d designating a duty D, based on an intensity signal p andthe current value signal i; and a square wave pulse generator 103 foroutputting a square wave pulse current R as the LED driving currentbased on the current value signal i and the duty signal d.

FIG. 2 is a cross-sectional view illustrating a structure of the LEDdevice 104 of Example 1. The LED device 104 is fabricated in thefollowing way. A GaN buffer layer (not shown), an n-type GaN layer 202,an In_(0.2)Ga_(0.8)N active layer 203, a p-type Al_(0.1)Ga_(0.9)Nevaporation prevention layer 204, a p-type Al_(0.1)Ga_(0.9)N upperevaporation prevention layer 205, and a p-type GaN contact layer 206 aresuccessively formed on a sapphire substrate 201. The resultingmulti-layer structure is dry-etched by reactive ion beam etching (RIBE)until a portion of the n-type GaN layer 202 is exposed. A nickel (Ni)film 207 is formed and patterned on part of the p-type contact layer206.

A gold (Au) electrode pad 208 is formed on the Ni film 207. Thereafter,a titanium (Ti) film is formed on part of the exposed surface of then-type GaN layer 202 and an aluminum (Al) film is formed on the Ti filmto provide an n-side electrode 209.

The above-described fabrication process of the LED device 104 will bedescribed below in more detail. The sapphire substrate 201 is heated ata temperature of about 1050° C. in an atmosphere of H₂ using an MOCVDapparatus for the purpose of subjecting the sapphire substrate 201 tosurface treatment. Thereafter, the temperature of the substrate 201 isdecreased to about 500° C. to form the GaN buffer layer (not shown). Inthis case, the thickness of the buffer layer is about 250 Å, forexample. The substrate temperature is increased to about 1020° C. andthe n-type GaN layer 202 having a thickness of about 4 μm is grown onthe GaN buffer layer. The substrate temperature is decreased to about720° C. and the non-doped or Si-doped In_(0.2)Ga_(0.8)N active layer 203having a thickness of about 30 Å is grown on the n-type GaN layer 202.Thereafter, the p-type Al_(0.1)Ga_(0.9)N evaporation prevention layer204 having a thickness of about 100 Å is grown on the In_(0.2)Ga_(0.8)Nactive layer 203 at the same temperature (i.e., about 720° C.). Thesubstrate temperature is increased to about 900° C. and the p-typeAl_(0.1)Ga_(0.9)N upper evaporation prevention layer 205 having athickness of about 100 Å is grown on the p-type Al_(0.1)Ga_(0.9)Nevaporation prevention layer 204. The growth temperature is preferablyequal to, but actually is allowed to fall within a tolerable range ofabout plus or minus 50° C. from, the temperature at which theIn_(0.2)Ga_(0.8)N active layer 203 has been grown. Thereafter, thesubstrate temperature is increased to about 1000° C. and the p-type GaNcontact layer 206 having a thickness of about 3000 Å is grown on thep-type Al_(0.1)Ga_(0.9)N upper evaporation prevention layer 205.

A resist is applied on the p-type GaN contact layer 206 followed bypatterning using a photolithography technique. Part of the resultantmulti-layer structure is dry-etched so that a portion of the n-type GaNlayer 202 is exposed. The n-type electrode 209 is formed on part of theexposed surface of the n-type GaN layer 202. After removal of theresist, the nickel (Ni) metal film 207 having a thickness of about 30 Åto about 100 Å is deposited on part of the p-type GaN contact layer 206by patterning using a photolithography technique. The gold (Au) padelectrode 208 having a thickness of about 4000 Å is provided in a waysimilar to that used when the metal film 207 is provided.

FIG. 3 is a graph showing a relationship between a driving current andan emission wavelength when a direct driving current is applied to theLED device 104 of Example 1. In FIG. 3, when DEVICE 1 is employed as theLED device 104, as the driving current is increased, the emissionwavelength of the LED device 104 is shifted toward a shorter wavelength.That is, a blue shift phenomenon is observed. For example, when thedriving current is about 0.1 mA, the emission wavelength is about 508 nm(green). When the driving current is about 1 mA, the emission wavelengthis about 494 nm (blue-green). When the driving current is about 10 mA,the emission wavelength is about 481 nm (blue).

Thus, the emission wavelength is shifted by about 20 nm or more toward ashorter wavelength in the case of a 10 mA drive as compared with thecase of a 0.1 mA drive. The wavelength shift toward a shorter wavelengthby about 20 nm or more causes visual observation of a different color.In general, for an LED device 104 having its emission wavelength rangein the visible spectrum, a change in emission wavelength caused by achange in driving current needs to be suppressed to about 6 nm or lessin order to maintain the same visual color observed.

The emission wavelength is herein defined as follows. Referring to FIG.4, the emission wavelength spectrum of the LED device 104 has a singlepeak and the half-value width of the emission wavelength is about 30 nm,resulting in an emission characteristic having a satisfactory colorpurity. In the LED device 104 of Example 1, the peak wavelength of theemission wavelength spectrum represents the color of light emitted fromthe LED device 104. The peak wavelength of an LED device is hereindescribed as the emission wavelength of the LED device. For the sake ofsimplicity, an emission wavelength is used as a parameter of a lightcolor.

FIG. 5 is a graph showing the emission power characteristics of the LEDdevice 104 of Example 1 when it is driven by a pulse current. Thehorizontal axis represents D×I (mA) where D is the duty D of the squarewave pulse current R and I is the current value thereof. The verticalaxis represents the emission power P (μW) of the LED device 104. Thepulse herein means a periodic pulse as shown in FIG. 6. Referring toFIG. 6, the current value I represents a current value (peak value) of apulse. The duty D is represented by T₂/T₁ where T₁ is the period of apulse and T₂ is the duration of a pulse. Therefore, the product D×I ofthe duty D and the current value I represents a time-average currentinjected to the LED device 104.

In Example 1, for the sake of simplicity of description of the principleof the present invention, it is assumed that the LED device 104 isdriven in a current region where the emission power is substantiallyproportional to the current value. In this case, the time-averageemission power of the LED device 104 is proportional to the averagecurrent injected to the LED device, resulting in a substantiallystraight line as shown in FIG. 5. That is, it is believed that theemission power P is substantially equal to or proportional to D×I.

In this case, it is preferable that when a viewer observes the LEDdevice 104, he or she does not sense a flicker. To this end, the periodT₁ is about 30 ms or less, more preferably about 10 ms or less, and theperiod T₂ is preferably as long as or longer than the recombination lifespan of a carrier in the active layer. In the case where the LED device104 includes an InGaN active layer, the pulse duration is about 0.2 nsor more, and more preferably about 1 ns or more.

The operation of the LED device of Example 1 will be described belowwith reference to FIGS. 1 through 6. In this case, an LED device 104indicated by DEVICE 1 in FIGS. 3 and 5 has an emission wavelength ofabout 490 nm (blue-green) and an emission power of about 100 μW, forexample. The emission power is used as a parameter of emissionintensity. The LED on-off circuit 1 externally receives a color signal c(=c₄₉₀) indicating that the designated emission wavelength of the LEDdevice 104 is about 490 nm and an intensity signal p (=p₁₀₀) indicatingthat the designated emission power of the LED device 104 is about 100μW.

Initially, the color signal c is externally input to the colorsignal-current value converter 102. In the converter 102, the colorsignal c is converted to the current value signal i in accordance withdriving current versus emission wavelength characteristics shown in FIG.3 (DEVICE 1). When the color signal c (=c₄₉₀) is input to the converter102, the current value corresponding to an emission wavelength of about490 nm is about 2 mA in accordance with the driving current versusemission wavelength characteristics shown in FIG. 3 (DEVICE 1). Theconverter 102 generates a current value signal i (=i₂) indicating thatthe designated current value I of a pulse which should be output by thesquare wave pulse generator 103 is about 2 mA and sends the signal i tothe calculation processor 101 and the square wave pulse generator 103.

The calculation processor 101 receives the intensity signal p and theabove-described current value signal i, and calculates a duty D inaccordance with the D×I versus emission power characteristics shown inFIG. 5 (DEVICE 1) where I is a current value and D is a duty.

As is seen from FIG. 5 (DEVICE 1), the product D×I of the duty D and thecurrent value I is 1 mA with respect to the intensity signal p (=about100 μW). As the current value indicated by the current value signal i isabout 2 mA, the duty D is equal to about 0.5. Therefore, the calculationprocessor 101 generates a duty signal d (=d_(0.5)) indicating that thedesignated duty D of the pulse which the square pulse generator 103should output is about 0.5, and sends the duty signal d to the squarepulse generator 103.

The square pulse generator 103 generates and outputs a pulse current Rhaving a current value I which is equal to 2 mA and a duty D which isequal to about 0.5, based on the current value signal i (=i₂) sent fromthe color signal-current value signal converter 102 and the duty signald (=d_(0.5)) sent from the calculation processor 101. The LED device 104is driven by the pulse current R.

In this way, light having a designated emission wavelength and emissionpower can be emitted even when an LED device having an emissionwavelength which is dependent on a driving current value is employed.

The case where an LED device 104 indicated by DEVICE 2 in FIGS. 3 and 5outputs light having an emission power of about 100 μW and an emissionwavelength of about 490 nm which are the same values as described above,will be described. As is seen from the driving current versus emissionwavelength characteristics in FIG. 3, the emission wavelength of DEVICE2 is shifted by about 6 nm toward a longer wavelength at each currentvalue as compared to that of DEVICE 1. As is seen from the D×I versusemission power characteristics show in FIG. 5, the product of thecurrent value I and the duty D of DEVICE 2 is shifted toward the lowerlight output side as compared to that of DEVICE 1.

When the color signal-current value signal converter 102 receives acolor signal c (=c₄₉₀), the converter 102 generates a current valuesignal i (=i₅) designating a current of about 5 mA corresponding to anemission wavelength of about 490 nm in accordance with the drivingcurrent versus emission wavelength characteristics in FIG. 3 (DEVICE 2),and sends the current value signal i to the calculation processor 101and the pulse generator 103.

The calculation processor 101 receives the current signal p (=p₁₀₀) andthe current value signal i (=i₅). As is seen from the D×I versusemission power characteristics show in FIG. 5, the product D×I of theduty D and the current value I is about 3 with respect to the intensitysignal p (=about 100 μW). As a current value indicated by the currentvalue signal i is about 5 mA, the calculation processor 101 calculatesthat the duty D is equal to about 0.6.

The calculation processor 101 generates a duty signal d (=d_(0.6))indicating that the designated duty D of the pulse which the pulsegenerator 103 should output is equal to about 0.6, and sends the dutysignal d to the pulse generator 103.

The pulse generator 103 generates and outputs a pulse current R having acurrent value I of about 5 mA and a duty D of about 0.6, based on thecurrent value signal i (=i₅) sent from the color signal-current valuesignal converter 102 and the duty signal d (=d_(0.6)) sent from thecalculation processor 101. The LED device 104 is driven by the pulsecurrent R. In this way, light having a designated emission wavelengthand emission power can be emitted by driving the LED device 104 with thepulse current R.

As described above, with the driving method of the present invention,the LED device having an emission wavelength which is changed with adriving current value can be driven so that the LED device outputs lighthaving a constant wavelength and power. When the LED devices havingemission wavelengths which are changed with driving current values aredriven at a predetermined driving current value, the emissionwavelengths vary among the LED devices due to variations incharacteristics of the LED devices, resulting in variations in colortones. Such a problem is solved by the present invention.

The relationship of the D×I versus the emission power shown in FIG. 5may be stored as a table or function in the calculation processor 101.The relationship between the current value versus the emissionwavelength shown in FIG. 3 corresponding to a target LED device may bestored in the color signal-current value signal converter 102.

Although the above-described pulse wave is a square wave in Example 1,the pulse waveform generated by the pulse generator 103 is not limitedto the square wave, but may be any form including a triangular wave.

The pulse wave may be generated in the following way: (1) the duty ismodified by changing the pulse duration while the period is maintainedconstant; (2) the duty is modified by changing the period while thepulse duration is maintained constant; or (3) the duty is modified bychanging the number of pulses in a predetermined period of time.

The pulses may be concentrated in the first half of a predeterminedperiod (a in FIG. 18), the pulses may be concentrated on the latter halfof a predetermined period (b in FIG. 18), or the pulses are distributedin the entire predetermined period (c in FIG. 18). That is, as long asthe average power of the driving current is constant, the shapes, thewidths or the number of individual pulses may be arbitrarily selectedusing a simple method. As described above, there are the various methodsfor generating pulses, but the present invention is not limited to thosemethods. In the present invention, other pulse generation methods may beused.

Example 2

In Example 2, the LED apparatus (including DEVICE 1) of Example 1 isemployed. An attempt is made, where the emission intensity of the LEDdevice 104 is temporally changed from about 50 μW to about 100 μW toabout 200 μW while the emission wavelength of the LED device 104 ismaintained constant. In such an attempt, the color signal c isconstantly equal to c₄₉₀, and the intensity signal p is changed from p₅₀to p₁₀₀ to p₂₀₀.

Since the color signal c is constantly equal to c₄₉₀, the colorsignal-current value signal converter 102 outputs the same current valuesignal i as that in Example 1, i.e., i=i₂.

The duty signal d output from the calculation processor 101 iscalculated in a procedure similar to that in Example 1. As the intensitysignal p is changed from p₅₀ to p₁₀₀ to p₂₀₀, the duty signals d(=d_(0.25), d_(0.5), and d₁), which cause the duty D to be about 0.25,about 0.5, and about 1, respectively, are sequentially sent to the pulsegenerator 103.

Thus, in such a procedure similar to that in Example 1, the LED device104 outputs light having a intensity which is changed temporally and aconstant wavelength, the intensity and the wavelength being externallydesignated.

In Example 2, although the LED device having an emission wavelength(color tone) which varies depending on a driving current value isemployed, the emission intensity can be solely changed while theemission wavelength (color tone) is maintained constant.

Such a driving method may be applied to a display apparatus whichexhibits a color tone by simultaneously employing a plurality of LEDdevices having different light colors. In this case, color shift can besuppressed. Alternatively, the driving method may be applied to adisplay apparatus in which a fluorescent is excited by the LED device sothat the fluorescent emits light. In this case, a reduction in emissionefficiency due to excitation wavelength shift, or color tone shift canbe prevented. Even when an LED device having a characteristic differentfrom the LED device in Example 2 is employed (there are variations in acharacteristic), the emission intensity can be solely changed while apredetermined light color is maintained in a way and for the reasonssimilar to those described in Example 1.

Example 3

In Example 3, a simplified variant of the LED on-off circuit of Example1 is provided. Referring to FIG. 7, the LED on-off circuit of Example 3is comprised of only a square wave pulse generator 103 a. It is only theintensity signal p that is externally input to the LED on-off circuit.The square wave pulse generator 103 a only modifies the duty of acurrent pulse which is to be supplied to the LED device 104 based on thevalue of the intensity signal p while maintaining the peak value of acurrent at a predetermined constant value. If a change in wavelength(blue shift) of the LED device 104 is controlled so that the range ofchange is about 6 nm or less, variation in the peak value of a currentdoes not substantially cause problems. In this case, the peak value of acurrent is regarded as being constant. When the peak value of a currentis set to the maximum of the range in which the reliability of the LEDdevice 104 can be secured, the dynamic range of the emission intensityof the LED device 104 can be most preferably maximized. Specifically,the peak value of a pulse current is preferably set to about 10 mA ormore and about 300 mA or less.

Even when the LED on-off circuit having such a simple configuration isemployed, the LED device 104 can output light having an emission powerin the range from about 0.1 mW to about 50 mW with respect to a pulseduty in the range from about 0.2% to about 100%. In this case, theemission wavelength of the LED device 104 is decreased by about 4 nm,i.e., from about 472 nm to about 468 nm. Such a change (i.e., about 4nm) is less than about 6 nm at and above which a change in emissionwavelength is visually recognized as a change in color.

Even when the calculation processor 101 of Example 1 using the colorsignal c and the color signal-current value signal converter 102performing color control are not employed, a change in the light colorof the LED device 104 due to the blue shift thereof can be significantlysuppressed by controlling the emission power of the LED device 104 inthe following way. A pulse having a constant peak value is input to theLED device 104 so that a change in the emission wavelength of the LEDdevice 104 is less than about 6 nm, and a pulse duty is changed.Thereby, a change in the light color of the LED device 104 can bereduced to an extent in which a change in color is not visuallyrecognizable.

When the peak value of a pulse current is set to the maximum of therange in which the reliability of the LED device 104 can be secured, thedynamic range of the emission intensity of the LED device 104 can bemaximized.

Example 4

In Example 4, the LED apparatus (including DEVICE 1) of Example 1 isemployed. An attempt is made, where the emission wavelength of the LEDdevice 104 is temporally changed from about 475 nm (blue) to about 485nm (blue to blue-green) to about 494 nm (blue-green to green) while theemission intensity of the LED device 104 is maintained constant at about60 μW. In such an attempt, the emission intensity signal p is constantlyequal to p₆₀, and the color signal c is changed from c₄₇₅ to c₄₈₅ toc₄₉₄.

The color signal-current value signal converter 102 converts colorsignals c (=c₄₇₅, c₄₈₅, and c₄₉₄) to current value signals i (=i₃₀, i₄,and i₁) designating current values of about 30 mA, about 4 mA, and about1 mA in accordance with FIG. 3 similar to Example 1.

The duty signal d output from the calculation processor 101 iscalculated in a procedure similar to that in Example 1. As the currentvalue signal p is changed from i₃₀ to i₄ to i₁, the duty signals d(=d_(0.02), d_(0.15), and d_(0.6)), which cause the duty D of the squarewave pulse current R to be about 0.02, about 0.15, and about 0.6,respectively, are sequentially sent to the pulse generator 103.

Thus, in such a procedure similar to that in Example 1, the LED device104 outputs light having a wavelength which is changed temporally and aconstant intensity, the intensity and the wavelength being externallydesignated.

With the driving method of the present invention, the light color of anLED device can be changed without changing the emission intensitythereof. Thus, when the light color of an LED device is changed usingthe method of Example 4, a current value is preferably changed by afactor of about 10 or more in order to change a color tone, and morepreferably by a factor of about 20 or more.

The driving method of Example 4 may be applied to a display apparatus.Since the emission wavelength can be changed using a single LED devicewithout changing the emission intensity. Thereby, a display apparatushaving a simple configuration and a significant visual effect can beachieved using the driving method of Example 4. The light color of anLED device can be continuously changed with ease by continuouslychanging a color signal which is to be input to an LED on-off circuit.

Further, in a combination of Examples 2 and 3, light having an arbitraryset of different wavelength and intensity values, e.g., about 490 nm andabout 200 μW, and about 475 nm and about 60 μW, can be emitted by DEVICE1, for example.

In Examples 1 through 4, for the sake of simplicity, the operation ofthe LED device, in which the current value is proportional to theemission power, is described. The present invention is not limited tosuch an LED device. There are a number of actual LED devices which havesuch a proportional relationship. FIG. 8 shows current versus emissionpower (instantaneous value) characteristics of such an LED device. InFIG. 8, the current value is not proportional to the emission power, sothat the graph is a curve. In this case, the D×I versus emission powercharacteristics as shown in FIG. 5 cannot be drawn with a single line,but a different line must be used for each current value I.

An LED device having the current versus emission power characteristicsshown in FIG. 8 is employed. Using the method of Example 1, the LEDdevice is caused to emit light having a wavelength of about 490 nm and apower of about 100 μW which are the same target values as that inExample 1.

The color signal c (=c₄₉₀) is transferred to the color signal-currentvalue signal converter 102 which in turn outputs a current value signali (=i₂) designating a current value of about 2 mA. As is seen from FIG.8, when the current value is about 2 mA, an LED device outputs lighthaving a power of about 300 μW (instantaneous value). Therefore, thecalculation processor 101 calculates a duty D which is equal to about ⅓(=100 μW/300 μW), and outputs a duty signal d corresponding to the dutyD. In the foregoing, it is considered that the duty D is proportional tothe emission power P. However, when heat release is insufficient, if theproduct D×I of the duty D and the current value I is increased, theemission efficiency is decreased due to generated heat.

In this case, a proportional relationship is not necessarily establishedbetween the duty D and the emission power P. Nevertheless, the emissionpower P is represented by a function having the duty D and the currentvalue I as parameters (or D is a function having P and I as parameters).The function is obtained by determining characteristics of an LED devicein advance. The calculation processor 101 calculates the function basedon the LED device 104 and outputs a duty signal d corresponding to theresultant duty D. Alternatively, a relationship between the emissionpower P, the duty D, and the current value I may be stored as a table.In this way, the driving method of the present invention can drive anLED device having current versus emission intensity characteristics arenot linear.

In Examples 1 through 4, the color of light is represented using thepeak value of a light spectrum as a parameter as described in Example 1.This is done for the sake of simplicity. This is not essential, however,to the present invention as described in Examples 1 through 4. Forexample, when a CIE standard chromaticity diagram is employed torepresent the light color, the essence of the present invention is notimpaired.

Further, in Examples 1 through 4, for the sake of simplicity, theemission intensity of an LED device is represented by the emission power(W), but another parameter may be employed. When a parameter in whichvisibility is taken into account, such as luminance (cd/m²), luminousintensity (cd), or luminous flux (lm), is employed, if an LED device isdriven in such a manner that the emission wavelength is changed whilethe emission intensity is maintained constant, apparent brightness isadvantageously not changed due to a change in the emission wavelength.In this case, however, the relationship between the product D×I of theduty D and the current value I and the emission power cannot berepresented by a single line on a graph. Nevertheless, the drivingmethod of the present invention can drive an LED device having currentversus emission intensity characteristics are not linear. To this end,the relationship among the emission power P, the duty D, and the currentvalue I is stored in the calculation processor 101 where the unit of theemission power P is different from that in Examples 1 through 4. Theessence of the present invention described in Examples 1 through 4 doesnot depend on whether the current versus emission intensitycharacteristics of an LED device are linear or nonlinear.

Further, in Examples 1 through 4, the specific LED devices (DEVICE 1 andDEVICE 2) are employed, but the present invention can be applied to anyLED device having a wavelength which is decreased with an increase incurrent (a blue shift occurs in which the difference in wavelengthbetween the minimum driving current and the maximum driving current setin a driving circuit is about 6 nm or more).

For example, in an LED device having almost the same configuration asthat of the LED device shown in FIG. 2, when the composition of an InGaNactive layer thereof is modified in such a manner that the current valueis changed from about 0.1 mA to about 100 mA, the emission wavelengthcan be changed from about 590 nm to about 530 nm. The present inventioncan also be applied to an LED device in which an InGaN active layer orInGaAlN active layer is provided on a conductive (preferably n-type) GaNsubstrate. When these LED devices are employed in the configuration ofExamples 1 through 4, an LED apparatus which emits light having apredetermined color from orange to yellow to yellow-green to green canbe achieved.

Example 5

FIG. 9 is a diagram illustrating an LED bullet according to Example 5 ofthe present invention. The LED bullet of Example 5 includes an LEDdevice 801 having a light color which is changed with a driving currentvalue and a fluorescent 802 provided in a light emitting direction ofthe LED device 801. Light output from the LED device 801 and lightoutput from the fluorescent 802 are mixed, and the mixed light is outputfrom the LED bullet. The LED device 801 is the same as that in Example1.

The operation of the LED bullet of Example 5 will be described below.FIG. 10 is a graph showing excitation and emission spectra when thefluorescent 802 is excited at about 460 nm. The peak of the excitationspectrum of the fluorescent 802 used in Example 5 is about 460 nm. Ithas been found that when the fluorescent 802 is irradiated with lighthaving a wavelength of about 460 nm, the fluorescent 802 emits lighthaving the emission spectrum shown in FIG. 10 with the most efficiency.In Example 5, the peak wavelength of the emission spectrum is about 630nm, which indicates red light. An example of a fluorescent having such acharacteristic is a YAG fluorescent.

It is assumed that the fluorescent 802 is excited by the LED device 801having the current versus emission wavelength characteristics shown inFIG. 3 (DEVICE 1). As a driving current supplied to DEVICE 1 is changedfrom about 0.1 mA to about 2 mA to about 30 mA, the emission wavelengthof DEVICE 1 is changed from about 508 nm (green) to about 490 nm(blue-green) to about 474 nm (blue), respectively.

FIG. 11 is a CIE standard chromaticity diagram. In FIG. 11, a lineindicated by DEVICE 1 represents the light color of DEVICE 1 when thedriving current value is changed from about 0.1 mA to about 2 mA toabout 30 mA. A point indicated by FLUORESCENT 103 represents the lightcolor of the fluorescent 802 which has the emission spectrum shown inFIG. 10. As is seen from the excitation spectrum of the fluorescentshown in FIG. 10, when the emission wavelength of DEVICE 1 is about 508nm, the driving current supplied to DEVICE 1 is small, so that theemission power is small. In this case, the emission wavelength islocated at an end of the excitation spectrum, so that the fluorescent802 is substantially not excited. In contrast, when the emissionwavelength of DEVICE 1 is about 474 nm, the driving current is large, sothat the emission power is large. In this case, the emission wavelengthis located in the vicinity of the peak of the excitation spectrum, sothat DEVICE 1 is significantly excited to emit light. Thus, the shorterthe emission wavelength of DEVICE 1, the more significantly thefluorescent is excited.

For example, in the LED bullet of Example 5, when the LED device 801 isdriven at about 0.1 mA, the LED device 801 emits light having awavelength of about 508 nm, but the fluorescent is substantially notexcited. In this case, the LED device 801 substantially solely emitslight (green) in the LED bullet.

When the LED device 801 is driven at about 30 mA, light (i.e., blue,about 474 nm) emitted from the LED device 801 and light (i.e., red,about 630 nm) emitted from the fluorescent 802 excited by light emittedfrom the LED device 801 are mixed, so that the mixed light is redviolet.

When the LED device 801 is driven at about 2 mA, an emission wavelengthof about 490 nm (blue-green) is obtained, even though the emission powerof the LED device 801 is not large, and the emission of the fluorescent802 is not very large since the emission wavelength is in the vicinityof an edge of the emission spectrum. Light emitted from the fluorescent802 and light (i.e., blue-green, about 490 nm) emitted from the LEDdevice 801 are mixed, so that the mixed light is white.

Thus, the light color of the LED bullet when the current value ischanged from about 0.1 mA to about 2 mA to about 30 mA is shown with aline indicated by EXAMPLE LED in the CIE standard chromaticity diagramof FIG. 11. Thus, when the current value is changed from about 0.1 mA toabout 2 mA to about 30 mA, the light color of the LED bullet of Example5 is changed from green to white to red violet, respectively.

As described above, in Example 5, the combination of the LED devicehaving a light color which is changed with a current and the fluorescentallows the LED bullet to achieve the change in the light color due to achange in current that is not obtained using only an LED device.

In Example 5, the specific fluorescent shown in FIG. 10 is employed.There are various types of fluorescents. Appropriate combinations of LEDdevices and fluorescents can lead to variations in light color due to achange in current.

Example 6

Similar to FIG. 9, the LED bullet of Example 6 includes an LED device801 having a light color is changed with a driving current value, and afluorescent 802 provided in a light emitting direction of the LED device801. Light output from the LED device 801 and light output from thefluorescent 802 are mixed, and the mixed light is output from the LEDbullet. The operation of the LED device 801 of Example 6 will bedescribed below.

FIG. 12 is a graph showing the current versus emission wavelengthcharacteristics of the LED device of Example 6. In FIG. 12, DEVICE 3indicates the LED device in which the emission wavelength is changedfrom about 450 nm (blue) to about 430 nm (blue) to about 415 nm(blue-violet) as the current value is respectively changed from about0.1 mA to about 2 mA to about 30 mA. DEVICE 3 can be obtained bychanging the composition of the InGaN layer of the LED device shown inExample 1 and FIG. 2. FIG. 13 is a CIE1960UCS chromaticity diagram. Achromaticity coordinate (u, v) in the CIE1960USC chromaticity diagramhas a relationship with the corresponding chromaticity coordinate (x, y)in the CIE standard chromaticity diagram, the relationship being:

u=4x/(−2x+12y+3), and

v=6y/(−2x+12y+3).

The UCS chromaticity diagram has a characteristic that the distancebetween two coordinate points having different colors corresponds to acolor difference visually recognized. In FIG. 13, a line indicated byLED device shows a change in the light color of the LED device ofExample 6 when a driving current supplied to the LED device is changedfrom about 0.1 mA to about 30 mA. In this case, (u, v) is respectivelychanged from about (0.25, 0.03) to about (0.19, 0.06). Therefore, thedistance between the coordinate points (color difference) is about 0.07.

FIG. 14 is a graph showing the excitation spectrum and emission spectrumof a fluorescent of Example 6. In FIG. 14, the peak wavelength of theexcitation spectrum is about 430 nm. When the fluorescent is irradiatedwith light having such a wavelength, the fluorescent emits light havingthe emission spectrum with the most efficiency. In FIG. 14, the peakwavelength of the emission spectrum is about 650 nm having a color whichis red. Such a fluorescent can be made of 6MgO.As₂O₅:Mn⁴⁺. In FIG. 13, acoordinate point (u, v) (=(0.55, 0.31)) indicated by FLUORESCENTrepresents the light color of the fluorescent of Example 6.

According to the excitation spectrum of the fluorescent shown in FIG.14, when the emission wavelength of DEVICE 3 is about 450 nm (0.1 mA),the emission power of the LED device is small, so that the fluorescentis substantially not excited. However, as the emission wavelength ofDEVICE 3 becomes shorter (e.g., 415 nm (30 mA)), the emission power ofthe LED device becomes larger while light emitted from the LEDapproaches the peak of the excitation spectrum of the fluorescent.

Therefore, the fluorescent is significantly excited to emit light. TheLED bullet emits blue light which is dominated by light emitted from theLED device when the LED device is driven at about 0.1 mA.

When the LED device is driven at about 30 mA, light (i.e., red, about660 nm) emitted from the fluorescent and light (i.e., blue-violet, about415 nm) emitted from the LED device are mixed. Since the visibility ofthe light color of the LED device is low, the light (i.e., red light,about 660 nm) of the fluorescent is dominant. Thereby, red is visuallyrecognized.

Further, when DEVICE 3 is driven at about 2 mA, the emission wavelengthof DEVICE 3 is about 430 nm, i.e., blue. In this case, whereas theemission power of the LED device is very large, the emission of thefluorescent is not very large. The light is mixed to be violet.

The light color of the LED bullet when the driving current value ischanged from about 0.1 mA to about 2 mA to about 30 mA is shown with aline indicated by LED BULLET in the UCS chromaticity diagram of FIG. 13.The color tone (u, v) of the LED bullet is respectively changed fromabout (0.25, 0.1) to about (0.53, 0.28). The distance (color difference)between the coordinate points is about 0.33.

Thus, when the current value is modulated, the color tone of the LEDbullet of Example 6 is changed more significantly than the color tone ofthe LED device used in Example 6. This is because the fluorescent usedin Example 6 is one having an excitation efficiency which issignificantly changed (by a factor of about 2 or more, more preferablyabout 5 or more) as the emission wavelength of the LED device ischanged.

As described above, an LED device having a light color which is changedwith a current value is combined with a fluorescent having an excitationefficiency which is significantly changed as the emission wavelength ofthe LED device is changed. The resultant LED bullet has a moresignificant change in a light color with a current value than a changein a light color of the LED device.

Example 7

Similar to Example 1, a light emitting apparatus according to Example 7of the present invention includes: a light emitting section for emittinglight in which a color of the light is blue shifted with a value of adriving current; and a driving section for driving the light emittingsection so that the light emitting section emits light having a desiredcolor and a desired intensity, by generating the driving current basedon a signal designating the desired color and a signal designating thedesired intensity and by applying the driving current to the lightemitting section. In description of Example 7, the light emittingsection serves as an LED bullet, and the driving section serves as anLED on-off circuit.

An LED apparatus according to Example 7 will be described below. In theLED apparatus, the LED bullet of Example 5 is driven using an LED on-offcircuit similar to that of Example 1. The basic structure of the LEDon-off circuit of Example 7 is the same as that shown in FIG. 1, but isnewly shown in FIG. 17. The LED apparatus of Example 7 differs from thatof Example 1 in that an LED device 164 is an LED bullet in which afluorescent is provided in a light emitting direction of an LED deviceas described in Example 5. Further, in FIG. 17, an LED on-off circuit 16receives a luminance signal k designating the emission intensity of theLED bullet 164 and a chromaticity signal u designating the light colorof the LED bullet 164, and outputs a pulse current R having a currentvalue of I and a duty of D. The LED bullet 164 is driven in accordancewith the pulse current R. The LED on-off circuit 16 includes: achromaticity signal-current value signal converter 162 for convertingthe chromaticity signal u to a current value signal i designating acurrent value I; a calculation processor 161 for calculating andoutputting a duty signal d designating a duty D based on the luminancesignal k and the current value signal i; and a square wave pulsegenerator 163 for outputting a square wave pulse current R as an LEDdriving current based on the current value signal i and the duty signald. The square wave pulse current R has a current value of I and a dutyof D.

FIG. 16 is a graph showing the driving current versus chromaticity (uvalue) characteristics of the LED bullet 164 of Example 7. The u valueis a value on the u axis on the USC chromaticity diagram which isdetermined based on the light color corresponding to the peak wavelengthof the emission wavelength spectrum of the LED bullet 164. For the sakeof simplicity, the u value is used as a parameter of the light color ofthe LED bullet 164.

As is seen from the excitation spectrum of a fluorescent shown in FIG.10, when the excitation wavelength is about 508 nm (0.1 mA), thefluorescent is substantailly not excited. As the excitation wavelengthbecomes shorter toward about 474 nm (30 mA), the fluorescent is moresignificantly excited. Therefore, when the LED bullet 164 is driven atabout 0.1 mA, light emitted from the LED device is substantiallydominant, so that light emitted from the LED bullet becomes blue.

When the LED device is driven at about 30 mA, light (i.e., red, about660 nm) emitted from the fluorescent and light (i.e., blue-violet, about415 nm) emitted from the LED device are mixed. In this case, since thevisibility of the light color of the LED device is low, the light (i.e.,red light, about 660 nm) of the fluorescent is dominant. Thereby, red isvisually recognized.

When the LED device is driven at about 2 mA, the emission of thefluorescent is not very significant and light emitted from thefluorescent is mixed with light (i.e., blue, about 430 nm) emitted fromthe LED device, so that light emitted from the LED bullet becomesviolet.

The light color of the LED bullet when the current value is changed fromabout 0.1 mA to about 2 mA to about 30 mA is shown with a line indicatedby LED BULLET in the UCS chromaticity diagram of FIG. 13.

FIG. 16 is a graph showing the relationship between the current valueand the u value. As is seen from FIG. 16, when the driving current isabout 0.1 mA, the u value is equal to about u_(0.25), when the drivingcurrent is about 1 mA, the u value is equal to about u_(0.37), and whenthe driving current is about 10 mA, the u value is equal to aboutu_(0.475).

FIG. 15 is a graph showing the luminance characteristics of the LEDbullet of Example 7. The horizontal axis represents the current value Iof the square wave pulse current R, and the vertical axis represents theluminance (mcd) of the LED bullet. A characteristic curve is shown foreach duty D. That is, FIG. 15 is a graph showing the relationshipbetween the luminance L and current value I of the LED bullet of Example7.

The pulse period T₂ is not particularly specified, however, it ispreferable that a viewer does not sense a flicker. To this end, theperiod T₂ is about 30 ms or less, more preferably about 10 ms or less.

The operation of the LED apparatus and the LED on-off circuit of Example7 will be described in detail with reference to FIGS. 14, 15, 16, and17. The LED device having the characteristics shown in FIGS. 15 and 16is operated so that the u value is equal to u_(0.37) and the emissionintensity is equal to about 100 mcd, for example. Thus, luminance isused as a parameter of the emission intensity. The chromaticity signal u(=u_(0.37)) and the luminance signal k (=k₁₀₀) designating the luminance(about 100 mcd) of the LED device are externally input to the LED on-offcircuit 17.

Initially, the chromaticity signal u is externally input to thechromaticity signal-current value signal converter 162. In this case,the chromaticity signal u is converted to the current value signal i inaccordance with the driving current versus chromaticity characteristicsshown in FIG. 16. When the chromaticity signal u (=u_(0.37)) is input tothe converter 162, the converter 162 outputs the current value signal i(=i₁) indicating that the designated current value of a pulse to beoutput by the pulse generator 163 is about 1 mA. This is because thecurrent value versus chromaticity characteristics shown in FIG. 16indicates a current value of about 1 mA with respect to a chromaticityof about 0.37. The current value signal i (=i₁) is sent to thecalculation processor 161 and the pulse generator 163.

The calculation processor 161 receives the luminance signal k and thecurrent value signal i. In the calculation processor 161, a duty D iscalculated in accordance with the luminance characteristics of the LEDbullet shown in FIG. 15. In this case, the luminance signal k is k₁₀₀,and the current value signal i is i₁. As can be seen from FIG. 15, theduty D corresponding to the luminance k (=about 100 mcd) and i₁ (=about1 mA) is about 0.5. The calculation processor 161 calculates that theduty D is about 0.5, and outputs to the pulse generator 163 a dutysignal d (=d_(0.5)) indicating that the designated duty D to be outputby the pulse generator 163 is about 0.5.

The pulse generator 163 generates and outputs a pulse current R having acurrent value of about 1 mA and a duty of about 0.5, in accordance withthe current value signal i (=i₁) sent from the chromaticitysignal-current value signal converter 162 and the duty signal d(=d_(0.5)) sent from the calculation processor 161, and the LED bullet164 is driven by the pulse current R to emit light.

In this way, the LED device dependent on the u value of the chromaticitycharacteristics with respect to the driving current can emit lighthaving the designate luminance and u value on the USC chromaticitydiagram.

Further, an LED device (having variations in characteristics in FIGS. 15and 16) different from the above-described LED devices can be made toemit light having a predetermined luminance and chromaticity as shown inExample 7.

Example 8

In Example 8, the LED apparatus of Example 7 is employed. An attempt ismade, where the luminance of the LED bullet 164 is temporally changedfrom about 10 mcd to about 40 mcd to about 100 mcd while the u value ofthe chromaticity characteristic of the LED device 164 is maintainedconstant at about 0.37 as in Example 7.

In this case, the chromaticity signal u is maintained constant atu_(0.37), and the luminance signal k is changed from k₁₀ to k₄₀ to k₁₀₀.

The chromaticity signal-current value signal converter 162 outputs thesame current value signal i (=i₁) as that described in Example 6.

The duty signal d output from the calculation processor 161 iscalculated in a procedure similar to that described in Example 6.Therefore, as the luminance signal k is changed from k₁₀ to k₄₀ to k₁₀₀,the duty signal d which is output to the pulse generator 163 isrespectively changed from d_(0.02), d_(0.1), and d_(0.42) designatingduties D which are equal to about 0.02, about 0.1, and about 0.42,respectively.

As a result, the LED bullet 164 is driven in accordance with thetime-varying luminance signals k and the constant chromaticity signal u(note that, in this case, light having a luminance of about 180 mcd ormore cannot be obtained).

In Example 8, although the LED device having an emission wavelength(color tone) which is changed with a driving current value is employed,the emission intensity can be solely changed while the emissionwavelength (color tone) is maintained constant. Even in LED devicesdiffering from the above-described LED devices in characteristics, theemission intensity may be solely changed while the light color ismaintained to be a predetermined color, using a similar method and forsimilar reasons as those described in Example 1.

When the above-described driving method is applied to a displayapparatus in which LED devices having different light colors aresimultaneously controlled so as to provide color tones, color shift canbe suppressed.

Example 9

In Example 9, the LED apparatus of Example 7 is employed. An attempt ismade, where the u value of the chromaticity characteristic of the LEDbullet 164 is temporally changed from about 0.25 to about 0.37 to about0.51 while the luminance of the LED device 164 is maintained constant atabout 40 mcd.

In this case, the luminance signal k is maintained constant at k₄₀, andthe chromaticity signal u is changed from u_(0.25) to u_(0.37) tou_(0.51).

The chromaticity signals u (=u_(0.25)u_(0.37), and u_(0.51)) areconverted by the chromaticity signal-current value signal converter 162to the current value signals i (=i_(0.01), i₁, and i₂₀) designatingcurrent values of about 0.01 mA, about 1 mA, and about 20 mA, inaccordance with FIG. 16 as described in Example 7.

The duty signal d output from the calculation processor 161 iscalculated in a procedure similar to that described in Example 7.Therefore, as the current value signal i is changed from i_(0.01) to i₁to i₂₀, the duty signal d which is output to the pulse generator 163 ischanged from d₁, d_(0.22), and d_(0.1) designating duties D which areequal to about 1, about 0.22, and about 0.1, respectively.

As a result, the LED bullet 164 is driven in accordance with thetime-varying chromaticity signal u and the constant luminance signals k.

With the driving method of the present invention, the light color of anLED device can be changed without changing the luminance thereof. Thus,when the light color of an LED device is changed using the method ofExample 9, a current value is preferably changed by a factor of about 10or more in order to change a color tone, more preferably by a factor ofabout 20 or more.

When the above-described driving method is applied to a displayapparatus, the u value of the chromaticity characteristic of a singleLED device can be changed without changing the luminance thereof. Theeyes of a human being can recognize that light having any wavelength hasthe same brightness, and therefore, significant visual effects can beachieved using a display apparatus having a simple configuration.Further, the light color of the LED device can be continuously changedwith ease by continuously changing color signals input to the LED on-offcircuit.

In Example 9, an LED bulletin which a fluorescent is excited by an LEDdevice is employed. The light color of the LED bullet can be changedfrom blue to red without changing the luminance by changing the value ofa current supplied to the LED bullet using the LED on-off circuit andthe driving method thereof described in Example 1. Such a range ofvariation in light color is larger than that obtained by a conventionalLED apparatus.

In Examples 1 through 9, the driving current supplied to the LED devicesis a periodic pulse current. The fundamental period of the drivingcurrent is preferably about 30 ms or less, more preferably 10 ns orless, so as to prevent a viewer from sensing a flicker. Further, thepulse duration is about 0.2 ns or more, more preferably 1 ns or more, asdescribed above so that the carrier density can be reliably controlled.

In Examples 1 through 9, the duty is changed while the current value ismaintained constant, or the current value is changed while the duty ismaintained constant. Nevertheless, the current value or the duty may bechanged if the color category of the resultant light color is notsubstantially changed (plus or minus 3 nm or less of a desiredwavelength).

In Examples 5 through 9, a fluorescent may be appropriately selected toobtain the desired excitation spectrum and emission spectrum.

Examples of a fluorescent for emitting red light include ZnS:Cu,LiAlO₂:Fe³⁺, Al₂O₃:Cr, Y₂O₃:Eu³⁺, Y(P,V)O₄:Eu³⁺, Y₂O₃:Eu, and acombination of Y₂O₃:Eu and Y₂O₃S:Eu.

Examples of a fluorescent for emitting orange light include ZnS:Cu, Mn,(Zn, Cd)S:Ag, ZnS:Mn, and (Sr,Mg,Ba)₃(PO₄)₂.

Examples of a fluorescent for emitting green light include ZnS:Cu, Al,LaPO₄:Ce³⁺Tb³⁺, Sr(S,Se):Sm, Ce, ZnSiO₄:Mn²⁺, βZnS:Cu, ZnS:Cu, Fe(Co),ZnS:PbZnS:Cu, and a combination of ZnS:Cu, Al, and Y₂Al₅O₁₂:Tb.

Examples of a fluorescent for emitting blue light include CaS:Bi,(Sr,Ca)₁₀(PO₄)₆Cl₂:Eu²⁺, SrS:Sm, Ce, Sr₂P₂O₇:Eu²⁺, βZnS:Ag,(Ba,Ca,Mg)₁₀(PO₄)₆Cl:Eu²⁺, and 3Sr₃(PO₄)₂.CaCl₂:E²⁺.

Examples of a fluorescent for emitting white light include ZnO:Zn,ZnS:AsZnS:Au, Ag, Al, Ca₂P₂O₇:Dy, Ca₃(PO₄)₂.CaF₂:Sb,3Ca₃(PO₄)₂.Ca(F,Cl)₂:Sb³⁺, 3Ca₃(PO₄)₂.Ca(F,Cl)₂:Sb³⁺, Mn²⁺, MgWO₄,3Ca₃(PO₄)₂.Ca(F,Cl)₂:Sb³⁺, and Mn².

Example 10

In Example 10, a display apparatus is obtained using the above-describedLED driving method or LED apparatus. FIG. 19 is a schematic diagramillustrating a display employing the above-described LED on-off circuitor LED on-off method. In FIG. 19, the LED on-off circuit of theabove-described examples is incorporated into a display 171.Alternatively, the LED on-off circuit may be separated from the display171 (i.e., the LED on-off circuit do not need to be incorporated intothe display 171).

In FIG. 19, LED devices 172 (or LED bullets) are provided along X and Ydirections on a plane so as to form a matrix. When the LED devices 172are driven by a conventional LED driving circuit, variations in thecharacteristics of each LED device 172 are significant. Therefore,variations in the color and brightness of light result in adeterioration in the picture quality of the display. In contrast, whenthe display apparatus shown in FIG. 19 includes the LED apparatusdescribed in Examples 1 through 9 or employs the LED driving methoddescribed therein, if the LED devices 172 are driven by the respectiveLED on-off circuit connected thereto, the LED devices having variationsin the characteristics are all allowed to emit the desired light colorand brightness. Thereby, the picture quality of an image provided by thedisplay apparatus can be improved.

According to the present invention, an LED apparatus having a lightcolor which is not changed even if the emission intensity is changed,can be achieved where the light color of an LED device included in theLED apparatus is changed with a change in a driving current value.

A plurality of LED devices which have light colors which are changedwith driving current values and which have variations in thecharacteristics can be driven so that the same emission wavelength andthe same emission intensity are obtained. Conventionally, when the LEDdevices having light colors which are changed with changes in drivingcurrent values are employed, the emission wavelengths (i.e., colortones) of the LED devices are different from one another due tovariations in characteristics thereof. Such a problem is solved by thepresent invention.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

1. A LED light emitting apparatus, comprising a plurality of LED devices which are located in a plane, of which the colors of the emitting light and the emission densities are varied, wherein each of the LED devices includes a fluorescent, the color of the emitting light of the LED device is adapted to have a blueshift according to a value of the driving current thereof, the LED device comprises a nitride semiconductor, and wherein in order to drive the LED device, a pulse current is loaded onto the LED device so that the peak value and the Duty ratio of the pulse current are separately controlled.
 2. A LED light emitting apparatus according to claim 1, wherein the pulse current has a period equal to or less than 30 ms and a pulse width equal to or larger than 0.2 ns.
 3. A LED light emitting apparatus comprising a plurality of LED devices which are located, wherein each of the LED devices includes a fluorescent, the color of the emitting light of the LED device is adapted to have a blueshift according to a value of the driving current thereof, wherein in order to drive the LED device, a pulse current is loaded onto the LED device so that the peak value and the Duty ratio of the pulse current are separately controlled, and wherein the pulse current has a period equal to or less than 30 ms and a pulse width equal to or larger than 0.2 ns.
 4. A LED light emitting apparatus comprising a plurality of LED devices which are located, and of which the colors of the emitting light are varied, wherein the color of the light of the LED device is adapted to have a blueshift according to a value of the driving current thereof, the LED device comprises a nitride semiconductor, and the LED device includes an active layer comprising InGaN, and wherein in order to drive the LED device, a pulse current is loaded onto the LED device so that the peak value and the Duty ratio of the pulse current are separately controlled.
 5. A LED light emitting apparatus according to claim 4, wherein the pulse current has a period equal to or less than 30 ms and a pulse width equal to or larger than 0.2 ns. 